When Did M-rods Begin?

[EricF]

I have heard conflicting reports of when Volvo started using the M-rods...

I have been considering putting my B21F (original motor) back in the GT, it's a 1978. I know it's the lower compression ratio engine with the dished pistons, think it was rated at 102 bhp or some similar pathetic figure.

Really though just wondering when M-rods started showing up. The engine is 3 hours away from me or else I would pull the pan and do some investigation. The GT thanks you for your information.

[thelostartof]

i thought all b21/b23's got M rods no matter what the year

[EricF]

From some thread on CIS:
I think the M rods came out in '77 or '78, if I'm not mistaken.


From Wagner's FAQ on redblocks...

All these engines have the same crankshaft and same con-rods. Engines from 1975-1980 have weaker con-rods than the newer engines, difference being in the material.

[thelostartof]

ah hm

[Wagner]

Yep, that's a good question. All the engines dating to 1980 and onwards that i've opened have had M rods.

But then the GT engine in 1979 did have M-rods as well.

But i've opened one 1978 B21 which had the similar looking rods as M-rods, but without the embossed M.

So i guess they were using old stock along the very late 70s, because there would've been no point in them having two different types of conrods made, moneywise that is.

On the subject of the difference in strenght. I do not have any personal experience using the pre M-rods, but this matter has been discussed probably since the 80s. I've read about it in a swedish forum sometime in the 90s. And in some old KG cataloques they advise against using those pre-M-rods.

Then there was this old finnish redblock tuner who said the same thing in the 90s when i was building my very first redblocks.

[EricF]

Yep, that's a good question. All the engines dating to 1980 and onwards that i've opened have had M rods.

But then the GT engine in 1979 did have M-rods as well.

But i've opened one 1978 B21 which had the similar looking rods as M-rods, but without the embossed M.

So i guess they were using old stock along the very late 70s, because there would've been no point in them having two different types of conrods made, moneywise that is.

On the subject of the difference in strenght. I do not have any personal experience using the pre M-rods, but this matter has been discussed probably since the 80s. I've read about it in a swedish forum sometime in the 90s. And in some old KG cataloques they advise against using those pre-M-rods.

Then there was this old finnish redblock tuner who said the same thing in the 90s when i was building my very first redblocks.

Very interesting info, Wagner, thank you.

Do you have any speculation as to how much weaker the pre-M rods were? Or know anyone who would know? I'm not building a balls-to-the-walls project, but it will likely have a 19T and 531 in the not so distant future, though it will still be limited by mechanical ignition timing.

[John V, outside agitator]

Very interesting info, Wagner, thank you.

Do you have any speculation as to how much weaker the pre-M rods were? Or know anyone who would know? I'm not building a balls-to-the-walls project, but it will likely have a 19T and 531 in the not so distant future, though it will still be limited by mechanical ignition timing.

You want some M rods?
What's the question?
Shirley you don't want the known weak Pre M rod things.

[EricF]

You want some M rods?
What's the question?
Shirley you don't want the known weak Pre M rod things.

Need an engine in Orlando to put in my GT this weekend, can't pull one at the yard and also expect to pull my old B21FT and put the other engine in the GT, all in a Saturday plus Sunday morning. If the donor engine is free-floating, it becomes do-able.

I have the 1978 B21F in my garage... If it has M-rods, it will be suitable for the car, if it does not then I think I would hesitate to put it in, even if it were only serving as an interim motor (especially with a 531 head on top, don't want a rod to part and a squish pad to be literally squished by a slug).

The idea is to see if I can get the thing back on the road for the SE meet... Which is fast approaching. Only have one free weekend to do so, and the car is 3 hours away.

A fool's predicament, but one I'm in no less!!!

[Mueller]

so you have to have the bigger rods for a near stock normally asperated motor?

why don't we hear of non-M rod motors throwing parts thru the block everyday, there still has to be a few thousand of them stilling running around town here and there.....

the motor in my '78 has not thrown a rod and it has 400K mile on the original parts (minus bearings)

[Morley]

Eh...when did finding a redblock with proper rods become a problem? Should be plenty of engines from between '80-'84 out there.

[EricF]

so you have to have the bigger rods for a near stock normally asperated motor?

why don't we hear of non-M rod motors throwing parts thru the block everyday, there still has to be a few thousand of them stilling running around town here and there.....

the motor in my '78 has not thrown a rod and it has 400K mile on the original parts (minus bearings)

Megasquirt, S60R injectors, 531 head with K cam, FMIC, exhaust, lots of boost (now a 15G but soon a 19T)...

So in spite of the mechanically set ignition curve, it should be capable of putting something short of an M-rod to its limit. I have been running 15-17 psi boost and 23-25 degrees of initial advance with the B21FT/15G/160 SCP head. With slightly higher boost levels and similar (or slightly lower) ignition timing with a 19T, 531, and a tad more compression, stress on the rods will be significantly higher than it already has been.

[Mueller]

Megasquirt, S60R injectors, 531 head with K cam, FMIC, exhaust, lots of boost (now a 15G but soon a 19T)...

So in spite of the mechanically set ignition curve, it should be capable of putting something short of an M-rod to its limit. I have been running 15-17 psi boost and 23-25 degrees of initial advance with the B21FT/15G/160 SCP head. With slightly higher boost levels and similar (or slightly lower) ignition timing with a 19T, 531, and a tad more compression, stress on the rods will be significantly higher than it already has been.


the way your 1st post is written it appeared that you are putting back in a stock normally asperated engine (B21F) and keeping that way...

[242Bleek]

Megasquirt, S60R injectors, 531 head with K cam, FMIC, exhaust, lots of boost (now a 15G but soon a 19T)...

So in spite of the mechanically set ignition curve, it should be capable of putting something short of an M-rod to its limit. I have been running 15-17 psi boost and 23-25 degrees of initial advance with the B21FT/15G/160 SCP head. With slightly higher boost levels and similar (or slightly lower) ignition timing with a 19T, 531, and a tad more compression, stress on the rods will be significantly higher than it already has been.

It'll be ok, im pretty sure volvo didn't use any ****ty weak rods before M rods. Since M-rods fit a B20/B30 crank, im almost positive they are all the same, strong, heavy overkill M rods that we all know and love

[towerymt]

Even if they aren't 'M', they're still beefy. PM Lawrence ("fidel"), he can tell you what came in the B20s. I think there was an 'S' rod.

[John V, outside agitator]

It'll be ok, im pretty sure volvo didn't use any ****ty weak rods before M rods. Since M-rods fit a B20/B30 crank, im almost positive they are all the same, strong, heavy overkill M rods that we all know and love

No the B20 rods were disgusting junk ka-ka.
"Turbo-luv" brought some by here once and the caps were thinner than sheeeeeut and had no rib or ribs for anybody's pleasure, they were WEAK.

If they carried those over to the OHC motors then I say I wouldn't touch em.

But whatever they are in OPs core motor he may not be able to just swap the rods as the early pistons in some were 22mm and the later 24mm.

Decisons decisions,

Which rod would Jesus use?:e-shrug:

[OttoB]

No the B20 rods were disgusting junk ka-ka.

Which rod would Jesus use?:e-shrug:

There are some early-B20 6-bolt rods which are known to be good and stronger than early-b20 8-bolt rods. Known as Bofors-rods.

Edit:

But that is only something that has only historic value. H-rods are so inexpensive today, that you spend almost same money fixing old junk with zero resale value.

End of Edit.


Forgiving rod?